In March 2011, the most violent earthquake in Japan's recorded history shuddered through that country's bedrock. The greatest geological disturbance, however, was recorded not at the quake's epicenter in Tohoku, but the earth beneath Mount Fuji, indicating the active volcano is now likely in what researchers call a "critical state."

In a study led by Florent Brenguier, professor of seismology at the Institute of Earth Sciences in Grenoble, France, researchers measured signals produced by the interaction of ocean swells and solid, subterranean earth, called "seismic noise." Fluctuations in this noise allowed Brenguier and his team to map disturbances in the bedrock beneath Japan. According to Brenguier, the greatest perturbations were measured in the regions beneath Mount Fuji, "the ones where the fluids trapped in the rock – boiling water, gas, liquid magma, which cause an eruption when they rise to the surface – exert the greatest pressure." Breguier says the seismic waves from the earthquake, the epicenter of which was roughly 400km away, added to this pressure, and therefore the disturbance.

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So should Japan be on alert – or, at least, a higher state of alert than the country's constant supervision of Fuji would suggest it already is? Brenguier says "yes" – though the capricious nature of volcanos ensures that his details are vague:

Our work does not say that the volcano will start erupting, but it does show that it's in a critical state...We cannot establish a direct relation of cause and effect between quakes and volcanic eruptions, even if statistically the former lead to an increase in the latter... All we can say is that Mount Fuji is now in a state of pressure, which means it displays a high potential for eruption. The risk is clearly higher.

More details on the study, and historical precedent for the link between earthquakes and volcanism, at The Guardian. Read the full study at Science.